1. Field of the Invention
The present invention relates to an inverter apparatus. More specifically, the present invention relates to an inverter apparatus based on the theory of PWM (Pulse Width Modulation) capable of supplying a load with an AC voltage of a predetermined frequency by turning on or turning off a plurality of switching elements.
2. Description of the Prior Arts
Conventionally, the inverter apparatuses of the PAM (Pulse Amplitude Modulation) system and the PWM system have been well known wherein a plurality of switching elements (for example, transistors, thyristors or the like) are connected to form a three-phase bridge and the time duration of ON and OFF of each switching element is controlled, and thereby a DC is converted into a three-phase AC output. The present invention aims particularly at improving the inverter apparatus by the PWM system.
FIG. 1 is a circuit diagram showing one example of a bridge circuit which is the background of the present invention and thereto the present invention is applicable. A bridge circuit 10 in this FIG. 1 comprises six switching transistors Q1 through Q6 connected to a DC power source DC, and each pair of these transistors Q1 through Q6 are connected in series, and these series connections are further connected to form a three-phase bridge. Then, a three-phase AC output is withdrawn from respective terminals U, V and W connected to the points of series connection of the switching transistors. Meanwhile, the switching transistors Q1, Q2, Q3, Q4, Q5 and Q6 are put in ON state (conductive state) when a voltage or a switching signal of high level is given through base terminals thereof X, Y, Z, X, Y, and Z, respectively.
For a first prior art of the inverter apparatus obtaining an AC output using the bridge circuit 10 by means of the PWM systems, for example, the description has been made as conventional technique in the Japanese Patent Application Laid-Open No. 46677/1982 laid open on Mar. 17, 1982. In this prior art, ON and OFF states during one cycle of AC (equivalent to 0.degree.-360.degree. in electrical angle) of the respective switching transistors Q1 through Q6 obtained by the PWM system are stored in a memory means such as ROM. Then, these ON or OFF signals are read out in sequence for a certain period of time, and switching signals are given to the base terminals X through Z of the respective transistors Q1 through Q6.
FIG. 2 is an address map of a ROM showing the switching signals given to the base terminals X through Z by means of this first prior art. Meanwhile, in this FIG. 2, only the switching signal based on the PWM system to be given to the base terminal X is illustrated, and the switching signals given to the rest of the base terminals Y through Z are omitted here because of the similarity of form.
In FIG. 2, in the addresses 0 through 511 of the ROM, combinations of the switching signals are stored which cause the bridge circuit 10 (FIG. 1) to generate an AC of the set frequency by means of the PWM system when a frequency from 1 Hz to 10 Hz is set, and in the addresses 512 through 1023, similarly, combinations of the switching signals are stored which cause the bridge circuit 10 to generate a frequency from 11 Hz to 20 Hz. In the address 2024 and thereafter, although illustration is omitted, combinations of the switching signals are stored which are used when setting a higher frequency than the above-mentioned. The reason for using a different combination of the switching signals on a frequency basis is to adapt it to the driving characteristics of the loads to be connected to the output terminals U, V and W in FIG. 1. Accordingly, if the driving characteristics of the loads are constant independent of the frequency, only the switching signal at any one of frequencies has to be stored.
For example, when an AC output of 1-10 Hz is outputted from the output terminals U, V and W, the ROM is addressed in sequence from 0 to 511 during a certain period of time, and thereby a high level or low level of state of the switching signal, that is, "1" or "0" is read out. In response to this state, controlling signals or switching signals are given to the base terminals X through Z of the switching transistors Q1 through Q6. In this case, if all of the switching signals are read out from the addresses 0 through 511 of the ROM in sequence, only the AC component of one cycle is obtained, and therefore this is required to be repeated to obtain a continuous AC output. Then, the period of read-out of the switching signals during one cycle (0.degree.-360.degree.) has to be determined according to the set frequency. That is, the period of the clock which is used to address the ROM in sequence is set appropriately according to the set frequency.
In the first prior art, the switching signals during one cycle for the switching transistors Q1 through Q6 are thus stored in the memory means. Accordingly, in order to enable the frequency to be selected arbitrarily, the switching signal has to be stored for each frequency range, and a memory element having a huge memory capacity is required to attain this purpose. Furthermore, in order to reduce ripples and harmonic components by improving the resolution or precision of the AC output, the capacity of memory element is required to be further increased.
In order to solve such a problem, a second method as shown in FIG. 3 has been proposed in the Japanese Patent Application Laid-Open No. 46677/1982 as quoted previously. This second method utilizes the fact that the three-phase sinusoidal waves whose negative parts are inverted can be expressed only by six values of waveforms (characteristics) within the range of 0.degree.-60.degree. in electrical angle which are formed by folding back at the position of 30.degree. in electrical angle.
That is to say, when the negative parts of the three-phase AC as shown in FIG. 3(A) are inverted by a phase inverter or the like, all the waveforms become positive as shown in FIG. 3(B). Considering by dividing into periods of 60.degree. in electrical angle, these waveforms as shown in FIG. 3(B) become quite the same although they have different phases in respective periods I, II, III, - - - , that is different phases correspond to one another. And, the waveform in the period of 60.degree. in electrical angle becomes axially symmetric at the middle point of each period, that is, at the position of 30.degree. in electrical angle as shown by one dotted line in FIG. 3(B). Then, by superimposing these waveforms while shifting from one another by 30.degree. in electrical angle, six characteristics D0 through D5 as shown in FIG. 3(C) are obtained. This, when taken in reverse, means that an ideal sinusoidal AC waveform is reformed by combining the six characteristics (waveform values) D0 through D5 as shown in FIG. 3(C).
For example, in the case of the U-phase, the characteristics D0 is selected in the range of 0.degree.-30.degree. in electrical angle, the characteristics D1 in the range of 30.degree.-60.degree. in electrical angle, the characteristics D2 in the range of 60.degree.-90.degree. in electrical angle, the characteristics D5 in the range of 90.degree.-120.degree., the characteristics D4 in the range of 120.degree.-150.degree. and the characteristics D3 is selected in the range of 150.degree.-180.degree. in electrical angle in sequence, respectively. And, by withdrawing these selected characteristics, a positive side waveform of the U-phase can be reformed. Then, to obtain a negative side waveform of the range of 180.degree.-360.degree. in electrical angle, the respective characteristics selected as described above have only to be inverted. Then, by shifting this waveform of the U-phase by every 120.degree. in electrical angle, a three-phase AC waveform can be obtained. That is, by continuing to combine the above-described characteristics D0 through D5 according to a predetermined rule, the three-phase AC can be reformed.
Thus, in the method proposed in the Japanese Patent Application Laid-Open No. 46677/1982, the above-described six characteristics, that is, the waveforms D0 through D5 are stored in advance to be read out arbitrarily. This method can reduce the memory capacity to a considerable extent compared with the first prior art, not sufficiently. That is, in this second prior art, a part itself of the waveform of the three-phase sinusoidal wave is stored, and therefore the reduction in the memory capacity has a limit in view of compatibility with the resolution.